In starters of the aforementioned type, synthetic resin materials have been used for several components which are subjected to impact loads in order to improve workability of the components and to absorb the impact loads caused during engagement between a pinion of the starter and a ring gear of the engine.
In, for example, JP(U)-B-2-7263, a starter for an internal combustion engine is proposed wherein an internal gear in the planetary gear is fashioned of a synthetic resin.
When the mechanical components of such starters, made of a synthetic resin, are subjected to a high temperature above a glass transition temperature of the synthetic resin, the mechanical strength of the synthetic resin is extremely reduced in comparison with subjecting the same to a low temperature. Starters for internal combustion engines are used in a temperature range of about -40.degree. C. to about 150.degree. C. Consequently, materials capable of withstanding the impact load caused during the operation of the starters, in particular, during the operation under elevated temperature conditions, must be used for the mechanical components of the starters.
However, the selected synthetic resins form mechanical components of the starters have been based upon an elastic modulus of the material, and no attention has been given to a relationship between a glass transition temperature of the synthetic resin materials and their mechanical strength during the operation of the starters.
For example, synthetic resin materials having a glass transition temperature of about 60.degree. C. have been used for the mechanical components of the starter, with no significant problems arising during a normal operating condition of the starter even at a high ambient temperature of an engine compartment of 150.degree. C., which ambient temperature occurs immediately after the engine is stopped, after, for example, a driving at a high speed under a high ambient temperature of greater than 50.degree. C. However, a problem does arise in that the mechanical components fashioned of such a synthetic resin were prone to damage due to an impact load when the high temperature condition was followed by an abnormal operating condition of the starter wherein an excessive impact load was caused by an abnormal engagement of the pinion and the ring gear due to, for example, an irregular key switch handling and an excessively early ignition.